Sources of inspiration:
In the original The Thing, the alien was a strong, tough, intelligent, bipedal carnivorous plant. Not very playable. See also the Doctor Who episode, Seeds of Doom. And Day of The Triffids.
DC Comics has Poison Ivy, The Floronic Man, and Swamp Thing. (I actually modeled a Geomancer after him.)
In one homebrew, I’ve also used Awakened trees (NPCs) and elves who used ritual magic to become “planty” (PCs)- in game terms the elves had a template.
Do you have a feel for how alien or familiar you want them to be? Or what kind of plant, or environment you want them to live in?